To the OP for future reference, if you lagered your beer for an extended period of time you might want to consider adding some dry yeast to your bottling bucket. Also, despite the fact that it makes sense for whatever yeast are left in suspension to work at fermentation (i.e. refrigerator) temperatures (provided you used a lager yeast strain), as others have said bumping the temp to around 70 for carbonating purposes (three weeks is the rule of thumb) won't have a downside but will help ensure you get the carbonation levels you are seeking.
As Yooper suggested, you should warm the beer up, get the yeast re-suspended if you can, and leave it at room temperature for a few more weeks.